1. Field of the Invention
The present embodiments relate to a protective layer, a composite for forming the protective layer, a method of forming the protective layer, and a plasma display panel including the protective layer. More particularly, the present embodiments relate to a protective layer made of magnesium oxide containing a predetermined amount of a rare earth element which has a short discharge delay time with little temperature dependency, a composite for forming the protective layer, a method of forming the protective layer, and a plasma display panel including the protective layer.
2. Description of the Related Art
Plasma display panels (PDPs) are self-emission devices that can be easily manufactured as large displays, and have good display quality and rapid response speed. In particular, because of their thinness, PDPs have received much interest as wall-hanging displays, like liquid crystal displays (LCDs).
FIG. 1 illustrates a PDP pixel. Referring to FIG. 1, sustain electrodes, each including a transparent electrode 15a and a bus electrode 15b made of a metal, are formed on a lower surface of a front substrate 14. The sustain electrodes are covered with a dielectric layer 16. The dielectric layer 16 is covered with a protective layer 17 to prevent a reduction in discharge and lifetime characteristics due to direct exposure of the dielectric layer 16 to a discharge space.
Meanwhile, an address electrode 11 is formed on an upper surface of a rear substrate 10 and is covered with a dielectric layer 12. The front substrate 14 and the rear substrate 10 are separated from each other by a predetermined gap. A space defined between the front substrate 14 and the rear substrate 10 is filled with an ultraviolet (UV)-emitting Ne+Xe mixed gas or He+Ne+Xe mixed gas under a predetermined pressure, for example 450 Torr. The Xe gas serves to emit vacuum UV (VUV) (Xe ions emit resonance radiation at 147 nm and Xe2 emits resonance radiation at 173 nm).
Generally, a protective layer of a PDP performs the following three functions.
First, a protective layer protects an electrode and a dielectric layer. Discharging can occur even when only an electrode or only an electrode and a dielectric layer are used. However, when only an electrode is used, it may be difficult to control a discharge current. On the other hand, when only an electrode and a dielectric layer are used, damage to the dielectric layer by sputtering may occur. Thus, the dielectric layer must be coated with a protective layer resistant to plasma ions.
Second, a protective layer lowers a discharge initiation voltage. A discharge initiation voltage is directly correlated with the coefficient of secondary electron emission from a material constituting the protective layer by plasma ions. As the amount of secondary electrons emitted from the protective layer increases, the discharge initiation voltage decreases. In this regard, it is preferable to form a protective layer using a material with a high secondary electron emission coefficient.
Finally, a protective layer reduces a discharge delay time. The discharge delay time refers to the physical amount describing the phenomenon in which discharging occurs at a predetermined time after a voltage is applied and can be represented by the sum of two components: formation delay time (Tf) and statistical delay time (Ts). The formation delay time is the time between when a voltage is applied and when a discharge current is induced, and the statistical delay time is a statistical dispersion of the formation delay time. The lower the discharge delay time, the faster addressing can be done for single scan. Further, a lower discharge delay time can reduce scan drive costs, increase the number of sub-fields, and improve brightness and image quality.
Various requirements that must be satisfied for a protective layer for PDP have been studied. For example, Japanese Patent Laid-Open Publication No. Hei. 10-167807 discloses MgO composite ceramics in which 1-20 wt % of Sc, Y, or La microparticles are dispersed in an MgO matrix, and a preparation method thereof. The MgO composite ceramics have been developed considering that MgO has good heat resistance, corrosion resistance, and insulating property but cannot be used as a structural material since it has poor strength, fracture toughness, and thermal shock resistance. That is, the patent publication provides MgO composite ceramics in which the microparticles are combined with a MgO matrix, which are improved in mechanical characteristics such as fracture toughness and thermal shock resistance, and a preparation method thereof. Table 1 of the patent publication presents evaluation results for relative densities and elastic strengths of MgO sintered bodies described therein.
Meanwhile, Japanese Patent Laid-Open Publication No. Hei. 10-231168 discloses a Sc—MgO composite ceramic sintered body in which Sc particles are dispersed in an amount of 1-20% by volume based on the sintered body. According to the patent publication, the ceramic sintered body has high strength and high fracture toughness and undergoes a decrease in strength reduction at high temperature.
Studies on the above-mentioned sintered bodies for protective layers have been focused on improvements in mechanical characteristics of the sintered bodies or protective layers formed using the same. However, since protective layers for PDPs may significantly affect the discharge characteristics of the PDPs, in particular, a discharge delay time and the temperature dependency of the discharge delay time, developments of new protective layers considering the discharge characteristics are needed.